Boston Marathon Bombing suspect Dzhokhar Tsarnaev, 19, is pictured in this undated FBI handout photo. Tsarnaev was indicted on charges of killing four people using weapons of mass destruction according to the US Federal prosecutor. [Photo/Agencies]

BOSTON - Accused Boston Marathon bomber Dzhokhar Tsarnaev was indicted by a federal grand jury on Thursday on charges of killing four people in the largest mass-casualty attack on US soil since September 11, 2001.

The 30-count indictment filed in Boston federal court charges the 19-year-old ethnic Chechen with setting off two homemade pressure-cooker bombs in a crowd of thousands at the race's finish line and with committing a carjacking and engaging in a gunbattle with police before his April 19 arrest.

Tsarnaev could be executed if convicted. His public defender, Miriam Conrad, declined to comment on the charges, which include use of a weapon of mass destruction, bombing a public place and carjacking during four days that traumatized the Boston area.

"Today's charges reflect the serious and violent nature of the event ... the defendant's alleged conduct forever changed lives," said Carmen Ortiz, the US Attorney in Massachusetts. She said she had met with several of the people who were wounded in the attack and relatives of those who were killed.

"We will do all that we can to pursue justice, not only on their behalf but on behalf of all of us," Ortiz said.

The April 15 bombing was followed by the shooting of a campus police officer in Cambridge, a carjacking and a late-night gunbattle with police in the nearby suburb of Watertown. Dzhokhar's 26-year-old brother Tamerlan died in the gunbattle, which led to a daylong lockdown of most of the Boston area.

That evening, Dzhokhar was found hiding in a boat in a resident's backyard and arrested after police fired a hail of bullets.

The brothers started preparing for the attack more than two months earlier, when Tamerlan traveled to a New Hampshire fireworks store to buy 48 mortar shells containing about eight pounds (3.6 kg) of explosive powder, according to the charges.

Three people died in the bomb attacks: 29-year-old restaurant manager Krystle Campbell, 23-year-old graduate student Lingzi Lu and 8-year-old Martin Richard. Days later, the pair killed a campus policeman in their attempt to escape arrest, the charges said.

The younger Tsarnaev was not present at the indictment, and Ortiz declined to comment on his condition or where he was being held. He was badly injured in the April 19 gun battle and had been held in a prison hospital west of Boston. He is scheduled to be arraigned on July 10.